Histopathological and electron microscopic studies of lung lesions induced in F344 rats by chronic BCNU treatment showed that the animals developed a severe interstitial fibrosis, emphysema, atelectasis, chronic bronchitis and peribronchitis as well as pneumonia. Macroscopically visible nodular lesions were identified as proliferated serous cells of peribronchial submucous glands accompanied by fibrosis and inflammation. Preneoplastic lesions (bronchial and bronchiolar hyperplasia, early squamous metaplasia, adenoma of peribronchial submucous gland and squamous metaplasia of proliferated submucous gland) were also found.